1. Field of the Invention
The present invention relates to a technique for evaluating recording of data on an optical disk.
2. Description of the Related Art
Optical disks such as write-once type Blue-ray discs (hereinafter referred to as “BD-R”) and write-once type HD-DVD discs (hereinafter referred to as “HD-DVD-R”) are constructed by forming a recording layer, a reflective layer, and a protective layer if necessary on one surface of a substrate in the form of a light-transmitting disk. Spiral or concentric grooves are formed on the surface of the substrate having a recording layer and a reflective layer formed thereon, and convex parts called lands are formed between adjoining grooves. Recording is performed on such an optical disk by irradiating the recording layer on the grooves with a recording laser beam from an optical disk recording and reproduction device so as to track the grooves with the beam, thereby forming pits on the recording layer. Reproduction is performed by irradiating an array formed by such pits which have a length nT (T representing the length of bits between reference channel clocks, nT representing a length that is n times the length T (n is an integer)) and intervals having a length nT between the pits (hereinafter referred to as “spaces”) and converting light reflected by the array into a reproduction signal.
An optical disk recording and reproduction device performing recording and reproduction in such a manner is designed to accommodate variation of recording conditions which can occur each time recording is performed on each optical disk because of, for example, the disk drive used, the optical disk (also called medium), and the recording velocity. In order to accommodate such variation of recording conditions, recording and reproduction devices as described above employ techniques for setting an optimum laser light intensity (hereinafter referred to as “recording power”). Referring to such techniques, some devices of this type employ OPC (Optimal Power Calibration) as one means for selection OPC involves test recording which is performed in a test area (power calibration area) of a recording disk with recording power varied prior to data recording. Then, results of the test recording are compared with initial conditions registered in advance to select and set best recording power which provides high recording quality. Recording in a data recording region of an optical disk is performed with a recording laser beam having the best recording power thus set. Then, various evaluation indexes are calculated from waveforms reproduced from recorded waveforms, the indexes serving as parameters indicating states of recording reflecting changes in recording and reproduction signals caused by varying recording power conditions. The best recording power is determined such that the values of the indexes agree or approach target values to perform optimal correction of recording.
This goal is achieved by various techniques which will be briefly described including examples of the use of techniques based on the PRML (Partial Response Maximum Likelihood) signal processing method. The PRML, signal processing method is to process imperfect frequency responses causing inter-symbol interferences with frequency responses which provide a distortion-free condition, and the method is used in combination with a maximum likelihood decoding technique to prevent any reduction in signal quantity by eliminating inter-symbol interferences.
For example, Japanese Unexamined Patent Publication JP-A-2004-335079 has disclosed a technique for setting a recording parameter which is optimal for the maximum likelihood decoding method. Specifically, a calculation is carried out to obtain a reliability value |Pa−Pb|−Pstd| of the result of maximum likelihood decoding of a region which corresponds to leading and end portions of recorded marks and in which maximum likelihood decoding can result in an error at a high probability. Such a calculation is carried out for each of a combination of a predetermined mark length and a space length immediately preceding the same and a combination of the mark length and a space length immediately following the same. A recording parameter providing an optimal edge shift position is obtained from results of such calculations, and recording is performed such that the recording parameter thus obtained is reflected.
Japanese Unexamined Patent Publication JP-A-2003-303417 has disclosed a technique for accurately optimizing a recording strategy without any influence of noises even when the recording is performed in a high density. Specifically, a pulse response is determined so as to minimize a difference between a reproduction waveform obtained by reproducing a recording pulse signal, which is recording data having a high frequency pulse superimposed thereon, recorded on an optical recording medium and a waveform obtained by performing a convolution calculation between the recording data and the pulse response. Thus, the recording strategy is optimized. At this time, the same recording pulse waveform is recorded three times or more in the same track of the optical recording medium, and an average of sampling values of a reproduced waveform is obtained for each sampling sequence, and the averages are used as data of the reproduced waveform. Since averaged data are used, it is possible to eliminate any influence of random noises on the reproduced waveform.
Further, Japanese Unexamined Patent Publication JP-A-2003-151219 has disclosed a technique relating to the evaluation of quality of a reproduction signal. Specifically, the technique utilizes a predetermined reproduction signal, a first pattern which is associated with the pattern of a signal waveform of the reproduction signal, and an arbitrary pattern (a second or third pattern) which is different from the first pattern and which is associated with the pattern of the signal waveform of the reproduction signal. First, a difference D=Ee−Eo between a distance Eo between the reproduction signal and the first pattern and a distance Ee between the reproduction signal and the arbitrary pattern is obtained. Next, a plurality of samples of reproduction signals is examined to obtain a distribution of such distance differences D. A quality evaluation parameter (M/σ) for reproduction signals is defined based on the ratio of the mean M of the distance differences D thus obtained to the standard deviation σ of the distribution of the distance differences D thus obtained. Then, the quality of the reproduction signal is judged from an evaluation index value (Mgn) represented by the quality evaluation parameter.
Japanese Unexamined Patent Publication No. JP-A-2003-141823 has disclosed a technique for evaluating the quality of a signal from an index which allows the error rate of a binarization result obtained using maximum likelihood decoding to be properly estimated. Specifically, let us assume a maximum likelihood decoding scheme which involves a plurality of states at a time k (k is an arbitrary integer) and employs state transition rules providing n (n is 2 or a greater integer) state transition lines that a transition from a state at a time k−j (j is 2 or a greater integer) to a state at the time k can follow, the scheme estimating the state transition line having the highest probability among the n state transition lines. Then, let us assume that PA represents the probability of a state transition from a state at the time k−j to a state at the time k following the state transition line having the highest probability among the n state transition lines; PB represents the probability of a state transition from a state at the time k−j to a state at the time k following the state transition line having the second highest probability among the n state transition lines; and |PA−PB| represents the reliability of a result of decoding performed from the time k−j until the time k. Then, the value |PA−PB| is obtained for a predetermined period of time or obtained a predetermined number of times. Variation of the value |PA−PB| is identified to obtain an index indicating the quality of a signal that is correlated with the error rate of a binarization result obtained using maximum likelihood decoding.
Further, Japanese Unexamined Patent Publication No. JP-A-2002-197660 has disclosed a recorded state detecting technique which makes it possible to detect the state of recording that depends on the channel when reproducing information recorded in a high density using a Viterbi detector. Specifically, a reproduction signal read out from a disk device is corrected by a band-pass filter and an equalizer to impart specific channel characteristics to the same, and the signal is thereafter read as a digital signal xi with an A-D converter at the timing of a synchronous clock generated by a PLL circuit. The signal xi is input to the Viterbi detector to obtain a Viterbi detection output signal. The Viterbi detection output is input to a reference level judgment device and an error margin calculation circuit. The error margin calculation circuit calculates a difference Ei between the digital signal xi and the Viterbi detection output and outputs the difference to a recording state detecting circuit. The recording state detecting circuit detects the amplitude or amplitude level and asymmetry of the difference using an output from the reference level judgment device and outputs the detected information.
Although there are various techniques for evaluating data recording as described above, those techniques have not been necessarily successful in establishing appropriate association between an evaluation of data recording as a whole and an evaluation of data recording of individual recorded patterns.
It is an object of the invention to provide a technique in which a novel evaluation index is employed to make a total evaluation of data recording.
It is another object of the invention is to provide a technique in which a novel evaluation index is employed to allow individual recorded patterns to be properly evaluated.
It is another object of the invention to provide a technique for allowing appropriate association to be established between a total evaluation of data recording and an evaluation of data recording of individual recorded patterns.
It is still another object of the invention to provide a technique for adequately adjusting recording conditions or recording parameters based on an evaluation of data recording.